Analog and digital integrated circuits (ICs) are typically designed and manufactured in silicon, GaAs, GaN or SiGe processes, which may be of the BiCMOS or CMOS type. These ICs can be mass-produced, for example, on 8 or 12 inch wafers. After production of a wafer, the wafer is diced to separate the ICs from each other, typically through sawing.
Dicing process is a relatively rough process: a diamond saw cuts through a wafer. As silicon is quite brittle, cracks may form relatively easily and extend into electronic circuitry. The electronic circuitry may then become defective: it is not capable of operating according to desired specifications, or even not capable of operating at all.
A standard way of preventing cracks forming into the electronic circuitry is by placing a “seal-ring” around all circuitry. A seal-ring is a ring consisting of (nearly) all doping- and metal-layers placed around the electronic circuitry. Silicon-nitride is usually placed as last layer on top of all circuitry (except bump-pads and bond-pads as they need to be electrically connected to the outside world) to prevent moisture getting into the silicon. The seal-ring also has an opening in the silicon-nitride as a silicon-nitride layer is also quite brittle (similar to glass), again to prevent cracks from entering the IC.
This seal-ring is therefore an essential part of the mass-production process of silicon chips in order to realize a high reliability and a high yield.
With integration of RF electronics on silicon, proposals of implementing antennas on silicon have come up. Antennas are used in all electronic equipment (from GSM, GPS, DECT, Bluetooth to radar systems) to convert electrical energy to electromagnetic energy and vice-versa. However, when these antennas are placed inside a seal-ring, the seal-ring can short the electromagnetic field of the antenna. This reduces radiation efficiency and affects the radiation pattern of the antenna. Some antenna-on-silicon designs simply ignore production process requirements by leaving out the seal-ring, resulting in lower yields or reliability problems or breakdown after some time in the application. Other designs implement antennas in special post-production processes thus increasing overall costs.
Modern IC processes require a minimum and maximum use of metal per given silicon-area for reproducibility of the etching process and the chemical-mechanical-polishing used in the back-end processing. Usually a defined metal-filling (called tiling) using minimum sized structures fulfils the metal density requirement without disturbing overall performance too much.
When antennas are placed on the silicon, testing becomes an issue as DC-voltage, output power and matching can no longer be measured.
When the abovementioned problems are solved, antennas can be integrated side-to-side a complete radar. Even if the antenna is not integrated on the silicon but in the package the overall “module” can be seen as a single device. Antennas usually have a size close to lambda/4 or lambda/2, where lambda is the wavelength in the material or in free-space. For 60 GHz the free-space wavelength is 5 mm, and antenna can thus be 1.25 mm or smaller pending the dielectric constant of the material.
Single antennas usually have a relatively wide radiation pattern, which can be as wide as ±60 degrees in azimuth and ±60 degrees in elevation. For some applications this beam-width is too wide. Narrower beam-widths can be realized with antenna arrays: multiple antennas in a row, column or matrix driven with the proper phase and amplitude for each antenna. This can be done on the silicon, but results in a new design (usually resulting in a much larger silicon area to accommodate the extra antennas) and thus a new, expensive mask-set. Economy of scales may be difficult to reach if the market consists of a large number of applications with low quantities. It would be much nicer if a low-cost, easy to mass-produce, auto-aligned structure can be made that allows defining the radiation pattern after the silicon is produced.
Parabolic dish antennas and horn antennas have been known to provide well defined gains and (narrow) beam-widths. However, a special heavy and expensive launcher used to launch the electromagnetic wave into a waveguide or horn antenna. For satisfactory performance the waveguide should also be aligned with the horn antenna.